The Metal Sphere at the Top of a Small Van

Multiple Choice

The metal sphere at the top of a small Van de Graaf generator has a radius of 8.0 cm.How much charge can be accumulated on this sphere before dielectric breakdown of the air around it occurs? (The dielectric strength of air is 3.0 MV/m.)

A) 2.1 µC
B) 3.3 µC
C) 1.3 nC
D) 6.7 µC
E) 4.2 µC